Exotic materials and jumbo-sized parts need extra attention
Workholding for large parts may bring to mind the classic line from the 1975 movie Jaws: “You’re gonna need a bigger boat.”
That’s because everything needs to be super-sized when machining workpieces like truck transmission housings, wind turbine blades, rocket bodies, and more, for industries like aerospace and defense, agricultural, energy, marine, rail, and on- and off-road transportation.
“Those of us from Vektek that go to manufacturing locations for various reasons have noticed that large parts are more commonly machined on CNC machine tools than previously,” said Lowell Martin, power workholding specialist, Vektek LLC, Emporia, Kan. “Larger machine tools are required, which takes more space, thicker concrete (floors) for the machines, and higher ceilings for part loading equipment such as gantries and load-assist equipment.”
Once the infrastructure Martin described is in place, manufacturers have to deal with rough parts that may be less than ideal.
“Large part workholding has some inherent issues,” said Larry Robbins, president of the commercial division for SMW Autoblok Corp., Wheeling, Illinois. “First of all, the base product is not very round to start with because it’s been cast or forged.”
Robbins said very few parts over 30-40" (762-1,016-mm) in diameter are machined out of a solid blank. Castings and forgings have become the norm, and the quality’s not always the best, so workholding has had to adapt to that.
“Train wheels come, and it almost looks like a caveman pounded them by hand,” he said of some of the workpieces he’s seen.
Not only the parts are large and heavy. So is the workholding.
SMW makes a train wheel chuck that’s 1,450 mm in diameter. The master jaw on the company’s 1,000-mm chuck weighs 45 lb (20.4-kg). Hainbuch’s 12" (304.8-mm), three-jaw chuck, which is a common size in many shops no matter the maker, weighs 146 lb (66-kg).
“So, you’ve got to have a bodybuilder standing in front of your chuck,” Robbins said.
Unless a young Arnold Schwarzenegger works in the shop, “large workpieces usually require that the workpiece be handled with a lifting aid or crane,” said Dean Winkel, a certified manufacturing technology sales engineer at J. Winkel & Associates Inc., Kitchener, Ontario, Canada. The firm represents Hainbuch America and other workholding manufacturers. “Loading, unloading and positioning the workpieces accurately can be difficult because of their size and weight.”
Because the larger parts also require proportionately larger workholding systems to accommodate the weight, inertia and cutting forces needed to process them, changing the workholding is also very involved and time consuming, Winkel said.
The extra effort and time to properly clamp the large workpieces that Winkel described are often overlooked, said Colin Frost, COO of Carr Lane Manufacturing Co., St. Louis, Mo.
“With large parts and many clamps, simply the time to clamp them can be long,” said Frost. “This can be exacerbated by the need to follow a specific clamping sequence or protocol to prevent part deformation.”
Garrett Gordon, senior engineer at Bluco Corp., Naperville, Illinois, said, “The size and weight of large parts and workholding components do factor into the overall time it takes to build a fixture. So, it’s important to find places in the process where you can save time without sacrificing quality. Whether that’s accessing CAD models on the floor to build against, or using a system that pins together faster, each minute you save on fixturing is another minute of run-time in the machine.”
Making Do or Making it Yourself
A perennial choice in workholding has always been whether to fixture on the fly or to invest in a solution designed for the situation. There are many factors to consider when deciding.
Without the advantages offered by a complete, off-the-shelf solution, the usual difficulties that shops face around workholding for large parts are the loss of efficiency and accuracy that come with trying to build a homemade fixture, or a fixture out of components that aren’t fully compatible, said Gordon. If a shop uses components that aren’t sized for the job and a fixture design not geared for what they’re machining, it won’t be able to capitalize on what its machine tool is supposed to be doing. The accuracy just won’t be there, Gordon said.
Winkel said shops lean toward versatility rather than having the ideal workholding because of the time and effort needed to change over, and the cost of part-specific workholding. The consequences are often reduced rigidity and support.
“Slower machining parameters are then used to compensate for the reduced rigidity,” Winkel said.
Reducing the machining parameters is a workaround, but it can result in unwanted effects, particularly in some of the more exotic materials used in aerospace. “If you slow down the machining you bring in work hardening,” said Robbins of SMW Autoblok. “High nickel, high cadmium content work hardens very easily. If you hold the part correctly, use the correct cutting tools and use the correct coolants, you don’t have that problem.”
When off-the-shelf workholding won’t do the job, custom workholding is an added expense that has to be factored into the budget in more ways than one.
“Custom-built workholding is expensive by virtue of ‘opportunity costs’,” said Carr Lane’s Frost. “What else could you be making in your machining center if you were not making special workholding? There are the actual costs of time and material too.”
Technology Evolving Big Time
According to Bluco’s Gordon, when people think of manufacturing technology, they think probing, automation, new software “and that kind of thing.” However, the most important technology manufacturers can add to their processes comes into play before an operator even turns the machine on, he said.
Not too long ago, the modular fixturing company saw one glaring issue that needed to be addressed, said Gordon. “It was the fact that a complete modular tooling solution specifically made for the largest parts out there just didn’t exist,” he said.
To remedy that deficit, the company designed and manufactured a full line of modular tooling specifically for extra-large parts. This fixturing is engineered to hold parts weighing up to several tons on base plates 20' (6.10-m) and larger. The new line is expected to be available commercially in the third quarter of 2021.
To give an example of the massive scale of these new extra-large modular components, Bluco’s standard size console block weighs 12 lb (5.44 kg). The same part, when sized up for the extra-large line, weighs in at over 100 lb (45.36 kg).
“It’s a system that pins together with ground surfaces to ensure you’re square every time,” Gordon said. “By using a right-sized, precision modular system, you can avoid a lot of the issues that required time-consuming workarounds before.”
In addition to new workholding that’s super-sized, manufacturers recently started offering fixtures that are smart. “Commonly, these types of applications require an output signal from a PLC to create a motion and then a confirmation input signal that it happened,” said Martin of Vektek. “This is useful with larger parts and difficult-to-machine materials because both are commonly machined from expensive castings where scrap is unacceptable.”
Vektek swing clamp products employed within workholding applications commonly convert the confirmation feedback signal to make it useful as an input to a PLC. Initially, compressed air regulated to low pressure and low flow is sent to single or multiple actuators within a common confirmation circuit. The company’s Air Sensing Control Kit has a digital pressure switch that can be adjusted to change state upon an expected pressure differential. The signal from the switch is the utility the PLC can understand.
Vektek has developed swing clamps in five sizes and several body styles with up to 7,500 lb (33-kN) of force with or without position feedback options. These are catalog standard products ready to go to work the moment they’re needed, Martin said.
The company has recently improved its fluid advance work support family with a newly released high-capacity product line. These are currently available in four sizes: 1,000, 2,000, 4,000 and 8,000 lb (4.45, 8.90, 17.79 and 35.59 kN) of support. All but the 1,000 offer a position-sensing feature that, coupled with the Air Sensing Control Kit, can provide a confirmation signal to the PLC. This provides the assurance that all work supports have advanced fully and complied to all of the surfaces of the casting in initial operations and machined surfaces in secondary operations, Martin explained.
Winkel, the Hainbuch representative, said, “A workholding changeover used to involve lifting the chuck into the machine using a sling or eye-bolts. The chuck’s bolts were then lined up with those on the spindle and carefully manipulated into position. The chuck was then indicated and tapped into place.”
The process can take hours, depending on the chuck’s size and weight and the accuracy level needed.
Faster, Easier, Safer Changeover
As a solution, several years ago Hainbuch started offering its CentroteX system, part of the company’s automated change line, that makes workholding changeover faster, easier and safer, Winkel said. All of the workholding devices are equipped with a common interface.
“The chuck is positioned using a lifting device that allows the workholder to be balanced horizontally and oriented radially on a rail,” Winkel said. “The interface connects the drawtube, aligns the workholding to micron-level precision and fastens it to the spindle using rapid-action bolts. The whole process takes a few minutes.”
SMW Autoblok has always made a large selection of large-diameter workholding. And new from the company is the MM e-motion electro-mechanical, universal four-jaw chuck, whose largest size is 1,250 mm. Lightweight versions of four of the five chuck sizes are available. The reduced chuck weight allows for higher workpiece weight. A lower chuck profile allows more usable Z stroke.
From SMW-Electronics, a new company created as SMW Autoblok started producing more technologically advanced chucks, are small electromechanical drives and controls that can transmit up to 1.5 kW over a 4-mm gap to allow an operator to control the chuck electrically. He then has the ability to control each jaw individually, enabling high-low clamping with no cylinder.
“If I get a bad casting in and it’s off-center by 4 mm, as long as I’m within the mechanical travel of the chuck it allows me to offset that part back to on-center or truly offset the part mechanically, rotating off-center to do different features of a part. This has never been possible in the past except with a very specialized workholding setup and very low rpms,” Robbins said. “These electronics not only give you the ability to move parts, they also allow you to measure clamping pressure, torque rotational value and jaw position.
“So now I have a live feedback (from which) I can give you all the information from doing a part from 30" [762 mm] in diameter down to an 8" [203.2-mm] part—possibly on the same chuck by just using different features of the system,” he added.
Not all improved products are dramatically changed, but the new offering can make a difference for customers.
Lang Technovation Co., Hartland, Wis., recently added a new solution for large parts to its Makro-Grip Ultra workholding line, which is designed for five-axis machining. The line’s new maximum standard size is 810 mm (31.89"), more than double the previous largest size of 355 mm (13.98"), said David Pynakker, facilities administrator.
Big and Exotic
Manufacturers in aerospace and defense, among others, make parts that are not only large, they’re also made of exotic materials. These materials include Inconel, 17-4 PH stainless steel and a variety of high-chromium alloys, among others. While giant workpieces present challenges on their own, adding exotic materials like those used in aerospace only add to the parameters needed to work with them.
“Aerospace materials require higher cutting forces, so the workholding’s rigidity is always a concern,” said Hainbuch representative Winkel. “Aerospace components are also machined in multiple steps in between periods of stress-relief.”
As metal is shaped into bars, plates, forgings or castings before precision machining, the initial shaping builds tension and compressive forces in the material, causing it to strain. When material is removed during machining, the remaining material will often release the strain—or stress—in undesirable ways, changing the shape of the metal. No matter the method that is used to relieve the stress in the material, the workpiece must be removed from the machine before it is machined again closer to its final size, Winkel explained.
Some manufacturers let a part sit in the machine tool while the stress eases, but more often changeover allows the machine to work on other components in the meantime.
Lang offers a solution that not only offers an alternative to dovetailing, it also saves wear and tear on clamps. Form closure technology, part of the Makro-Grip series, involves stamping indentations on the workpiece to match the gripping serration of the clamping jaws. This enables the highest holding forces and maximum process reliability at a very low clamping pressure, according to Lang’s website.
“Many Lang customers know about the stamping process. This involves workpiece preparation that not only preps your part in seconds instead of typical prep times like with dovetailing, it also limits your material waste (compared with dovetailing),” said Pynakker. “This is achieved by clamping on just 3 or 5 mm of material, allowing our Makro-Grip or Mark-Grip-Ultra vises to grip into those stamped parts for full tooth insertion for better holding power.”
Lang’s Makro•Grip technology is designed for five-sided machining of blanks as well as automated manufacturing.
The stamping process allows up to 20 tons of hydraulic pressure to prep the workpiece for Operation 1 with materials up to 45 HRC, which encompasses materials such as titanium and Inconel. This practically eliminates any part deformation, which is crucial to clamping flat and deformation-sensitive material and is a key factor in achieving the desired milling quality.
Carr Lane Manufacturing’s Frost said, “Even greater force is required when dealing with these materials.” The company has a full line of large-edge manual clamps that deliver massive clamping forces, including its new Pivoting Edge Clamps that are available in standard and low-profile versions and can exert more than 4,000 lbf (17.79 kN) of force per clamp.
The pivoting nose on the large edge clamps has a serrated jaw that clamps forward and downward simultaneously. It can be used to clamp against a fixed locator or against a matching downthrust backstop. Its slotted base provides adjustability.
In addition, to support very large workpieces where clamping time is greater, the Pivoting Edge Clamps can be easily converted to hydraulic operation.
“To convert the Pivoting Edge Clamp, simply remove the thrust screw and replace it with the Roemheld hydraulic cylinder and feeder cap,” said Frost. “Then a hydraulic line is connected to the feeder cap. While the clamp is not generally larger than in its manual version, there is a hydraulic line to deal with (inside the machine tool’s work area).”
The Roemheld hydraulic cylinder is suitable for automated operations. Cylinders are available in forces up to 35,000 lbf (155.69 kN).
According to Bluco’s Gordon, rigidity is important so as not to fracture inserts. “If your locating surfaces can’t stand the abuse the part itself delivers, it’s not going to accurately locate that part. You need hardened, rigid locating components to maximize accuracy and quality during machining,” he said.
Special accommodations can be made for more delicate materials. “The issue here is when you’ve got finished parts with surfaces that can’t be marred,” he said. “You may be dealing with a huge part that needs to be held securely, but it can’t be scratched or dinged in the process because it’s already finished or ground. If you have materials softer than the workholding system you’re using, sometimes you’ll see aluminum liners or soft jaws and locators. Those will protect the finished surfaces from damage.”